1. Field of the Invention
The subject invention relates to polyoxyethylene polyethers which are thermally stable at high temperatures. These polyethers find special utility as heat transfer fluids and soldering auxiliaries.
2. Description of the Related Art
In the past few decades the printed circuit board has evolved from a rare and expensive commodity to a virtually ubiquitous and inexpensive method of constructing electronic circuits. The development of the transistor, the accompanying miniaturization of other electronic components, and the increasing complexity of the circuits themselves has resulted in electronic circuits of high component density. Due to the high component density and the need for rapid, mass-production techniques, the technique of hand soldering components has largely been superseded by more modern and efficient soldering methods.
In the solder pot method, for example, the circuit board, the components mounted thereon, is partially immersed into a pot of molten solder, bonding the component leads to the foil conductors on the surface of the board. One of the disadvantages of the solder pot technique is the trapping of gas bubbles or debris under the surface of the board preventing the adherence of solder at these locations. A further disadvantage is the formation of an oxide coating, or dross, on the surface of the solder bath as a result of oxidation of the hot molten solder by atmospheric oxygen. The dross interferes with uniform soldering resulting in a high rejection rate of the finished boards.
In U.S. Pat. No. 2,740,193, an improvement to the pot soldering technique is disclosed wherein the previously soldered board is immersed into a second solder bath which is covered with a liquid layer of organophosphorus compound. This liquid, acting as a heat transfer fluid causes the solder to remelt. Agitation of the board in the heat transfer fluid and in the underlying solder bath enables the elimination of solder bridges between foil conductors. However, this procedure is less than satisfactory due to the high cost of the organophosphorus compound.
U.S. Pat. No. 3,054,174 describe the use of a high boiling fluid to bond leads to semi-conductor devices by immersing the devices with pre-positioned leads into the fluid. Suggested fluids are anhydrous lanolin, silicones, glycerine, ethylene glycol and polyethylene glycol. U.S. Pat. No. 3,214,827 discloses a process for soldering stacked printed circuit boards by immersion of the assembled, solder-coated components into a hot bath of fluid where solder reflow takes place. Corn oil is suggested as a suitable fluid.
U.S. Pat. No. 3,690,943 discloses the use of a stationary wave of heat transfer fluid to alloy two or more metals on a printed circuit board. The hot fluid, which may be selected from paraffins, fats, and mineral and vegetable oils, is pumped smoothly through a long but narrow orifice thereby forming a uniform wave of fluid. The previously metallized circuit board is passed through the wave at a speed appropriate to raise the temperature to a sufficient level so as to alloy the metals coated thereon.
The wave soldering technique has become quite important commercially. In wave soldering, a stationary wave of solder is formed in much the same manner as the wave of heat transfer fluid described in U.S. Pat. No. 3,680,943. The circuit board and mounted components are passed at a predetermined speed through the stationary wave of solder. The dross formed on the solder may be skimmed off as the solder is recirculated but nevertheless causes higher than desirable reject rates. To minimize dross and to decrease the number of unsoldered areas and solder bridges, thermally stable oils may be added to the wave soldering apparatus either in bulk or in continuously metered amounts. In addition to minimizing dross formation, these oils also have the advantage that they lower the surface tension of the solder, promoting more even solder coating.
All of the processes described above utilize a high temperature liquid, generally termed an "oil" irrespective of its actual composition. Regardless of the particular application, the oil must fulfill several, often conflicting requirements. First and foremost, it must have superior thermal properties. Among the thermal properties most desirable are high boiling point (and correspondingly low volatility), high smoke point, and in particular, high thermal stability with regard to common high temperature chemical reactions such as pyrolysis and oxidation. Further, it should not deposit appreciable amounts of resinous residue upon the circuit boards, or produce large amounts of such residue, or sludge, upon long-term use.
In addition to the thermal properties just mentioned, the oil should be non-reactive with the solder, the circuit board composition, and the electronic components. Finally, it is most desirable that the oil by easily and virtually completely removable by washing the inexpensive solvents, most preferably, water.
Polyoxyalkylene polyethers have been proposed for these uses. For example, polyoxyethylene glycols were proposed for use in U.S. Pat. No. 3,054,184, discussed previously. However, despite being relatively inexpensive and water rinseable, these polyethers suffer from low thermal stability and therefore have not been used to any appreciable extent. Alkylphenol oxyethylates have been utilized by the industry, but still do not have thermal stability of the degree desired. An oil of improved thermal stability was disclosed in U.S. Pat. No. 4,360,144. These improved oils are heteric polyoxyalkylene polyethers initiated with bisphenol A, containing a 1:3 ratio of oxyethylene and oxypropylene residues in the polyoxyalkylene chains. Although these materials constituted a considerable advance over oils previously available in terms of thermal stability, further improvement is desirable. Furthermore, the bisphenol A initiated polyethers had less than the optimal degree of rinseability.